In a technique known as companion planting the three crops are planted close together. Flat-topped mounds of soil are built for each cluster of crops.[1] Each mound is about 30 cm (12 in) high and 50 cm (20 in) wide, and several maize seeds are planted close together in the center of each mound. In parts of the Atlantic Northeast, rotten fish or eels are buried in the mound with the maize seeds, to act as additional fertilizer where the soil is poor.[2] When the maize is 15 cm (6 inches) tall, beans and squash are planted around the maize, alternating between the two kinds of seeds. The process to develop this agricultural knowledge took place over 5,000–6,500 years. Squash was domesticated first, with maize second and then beans being domesticated.[3][4] Squash was first domesticated 8,000–10,000 years ago.[5][6]

The three crops benefit from each other. The maize provides a structure for the beans to climb, eliminating the need for poles. The beans provide the nitrogen to the soil that the other plants use, and the squash spreads along the ground, blocking the sunlight, helping prevent the establishment of weeds. The squash leaves also act as a "living mulch", creating a microclimate to retain moisture in the soil, and the prickly hairs of the vine deter pests. Corn, beans, and squash contain complex carbohydrates, essential fatty acids and all eight essential amino acids, allowing most Native American tribes to thrive on a plant-based diet.[7]

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Corn, squash and beans were planted ca. 800 AD in the largest Native American city north of the Rio Grande known as Cahokia, in what is now known as the US state of Illinois, across the river from St Louis, Missouri. The Three Sisters crops were responsible for the surplus food that created an expanded population throughout the extended Mississippi River valley and tributaries, creating the Mississippian and Mvskoke cultures that flourished from ca. 800 ce to ca. 1600 when physical contact with spanish explorers brought european disease, death and cultural collapse.

The division of labor among the Iroquois and Seneca peoples was noted to be that women tended the crops, the 'three sisters'. This was because men could be absent from their home and villages for an extended amount of time while hunting, attending diplomatic missions and raiding. The initial preparation for the planting of the 'three sisters' was performed by the men who cleared the land. After the land was cleared, groups of women who were related to each other would then do the planting, weeding and harvesting.[12]

^Bushnell, G. H. S. (1976). "The Beginning and Growth of Agriculture in Mexico". Philosophical Transactions of the Royal Society of London. London: Royal Society of London. 275 (936): 117–120. doi:10.1098/rstb.1976.0074.